The present invention relates to a method of and an apparatus for deinking printed wastepapers.
In accordance with well-known methods, printed wastepapers have hitherto been deinked in such a way that the feedstock has been exposed to an action of mechanical power, chemicals and heat, particularly in aqueous media. The actual removal of released printing inks from a fiber suspension or pulp has been carried out by washing or by flotation. In the prior art there have existed a large number of processes differing from one another by amounts and types of deinking chemicals used. All processes, however, have been characterized by common basic disadvantages resulting from the very methods of separating printing inks by washing or by flotation.
An essential problem to face in deinking processes resides, in general, in that the released particles of printing inks have to be prevented from being resorbed by fibers, the resorption taking place due to the fact that the aforesaid processes have been carried out in strongly alkaline media and that the fibers soaked with alkaline possess a relatively high adsorptive power. In the deinking processes, the printing inks are usually released within a very broad range of particle sizes, to wit, from the prevailing finest particles up to coarse grained ones which latter are visible with naked eye. It is why the separation of printing inks once released is rather difficult, since it is impossible to separate particles finer than 2 microns from the fiber by flotation. In addition, the flotation methods of separating printing inks, as a rule, lay excessive claims on industrial installations, in view of the size and number of the requisite flotation cells necessary to obtain desirable effects.
Another disadvantage of the flotation processes consists in a technically rather elaborate preparation of an air-dispersion in the flotation cell as well as in a complicated attendance resulting from a considerable sensitivity of the respective processing plant, which depends upon the necessity of absolutely homogeneous dosing rates and upon composition of the wastepapers feedstock. Every change in the composition of the respective wastepaper batch, requires a selective readjustment of the flotation plant as well as of the overall process.
On the other hand, deinking methods wherein the released printing inks are separated by washing, are connected with the generation of a considerable waste water volume and with troubles in processing highly diluted sludges of which removal is usually very elaborate and uneconomical. Any discharge of waste waters left after washing which is fed immediately into recipient water courses without prior liquidation of sludges is forbidden by law.
The purpose of the present invention and the basic object of the same is to overcome the aforementioned disadvantages and to significantly improve the deinking of printed wastepapers.